Polyesters, especially polyalkylene terephthalates, have excellent physical and chemical properties and have been widely used for resins, films and fibers. For example, polyester fibers have relatively high melting points and can attain high orientation and crystallinity. Accordingly, polyesters have excellent fiber properties such as chemical, heat and light stability, and high strength.
However, polyesters, especially polyester fibers, are difficult to dye. The molecular structure and the high levels of orientation and crystallinity that impart desirable properties to the polyester also contribute to a resistance to coloration by dye compounds. Also contributing to the difficulty in dyeing polyester is the absence in polyesters of ionic dye sites, in contrast to protein fibers, for instance.
In order to make a polyester dyeable by cationic or basic dyes, the polyester must be modified by incorporating dye sites. The most common method to incorporate such sites is polymerization in the presence of either a dimethyl ester or a bis-ethylene glycol ester of 5-sodium sulfoisophthalic acid. The bis-ethylene glycol ester of 5-sulfoisophthalic acid is generally prepared by transesterification of the dimethyl ester of the sodium salt of 5-sulfoisophthalic acid using excess ethylene glycol at 160-250.degree. C. and an ester interchange catalyst. U.S. Pat. Nos. 3,936,389 and 5,607,765 disclose the preparation and utility of bis-ethylene glycol ester of 5-sulfoisophthalic acid in modified polyesters. U.S. Pat. No. 3,936,389 discloses that a mole of the dimethyl ester of a metallo sulfodicarboxylic acid reacts with up to 30 mole equivalents of ethylene glycol, but only 2 moles of the glycol actually react, the remaining glycol acts solely as a solvent for the product. U.S. Pat. No. 3,900,527 discloses the preparation of a bisglycol ester of 5-sulfo isophthalic acid, sodium salt for incorporation into polyesters to improve dyeability and affinity for basic dyes.
U.S. Pat. No. 4,665,153 and JP 10287814 also disclose the preparation of a copolyester by reacting together a dicarboxylic acid, a glycol, a metal sulfonate, and a polyether glycol. The distribution of the monomers in such products is essentially random.
The melt viscosities of the copolyesters are substantially increased as the amount of sulfonate salts rises, resulting in low molecular weight polymers and difficulties in spinning process. It is known that polyether glycols, when used as a block comonomer, yield polyesters with lower melt viscosities and, therefore, can be added along with sulfonate salts during polymerization process to offset the high melt viscosities. Datye, in Colourage, 7-12, February 1994, and Gries et al., in Chemical Fibers International, Vol. 48, 508-513, December 1998, disclose incorporation of additives into polyesters. The polyether glycols are generally obtained by ring opening polymerization of cyclic ethers. The most common polyether glycols that are being used are poly(ethylene glycol), poly(l,2-propylene glycol) and poly(tetramethylene glycol).
It is therefore desirable to develop a composition that can be used to produce ether esters that improve the dyeability of polyesters and a process for producing the composition. An advantage of the invention is that the need to separately make the bis-glycol ester of sulfoisophthalic acid, or salt thereof, and the polyether glycol can be eliminated. Another advantage of the invention is that the preparation of ionic polyether glycols can be carried out in a single step.